1. Field of the Invention
In general, the present invention relates to a method of producing a lithographic mask (reticle) for use in the semiconductor industry. In particular, the invention pertains to the use of a deep ultraviolet (DUV) photoresist in combination with at least one antireflective coating (ARC) to produce a high performance mask. The invention also relates to use of an optical direct write continuous laser mask writing tool in combination with a chemically amplified DUV photoresist and an organic ARC.
2. Brief Description of the Background Art
Photoresist compositions are used in microlithographic processes for making miniaturized electronic components, such as in the fabrication of semiconductor device structures. The miniaturized electronic device structure patterns are typically created by transferring a pattern from a patterned masking layer overlying the semiconductor substrate rather than by direct write on the semiconductor substrate, because of the time economy which can be achieved by blanket processing through a patterned masking layer. With regard to semiconductor device processing, the patterned masking layer may be a patterned photoresist layer or may be a patterned xe2x80x9chardxe2x80x9d masking layer (typically an inorganic material or a high temperature organic material) which resides on the surface of the semiconductor device structure to be patterned. The patterned masking layer is typically created using another mask which is frequently referred to as a photomask or reticle. A reticle is typically a thin layer of a metal-containing layer (such as a chrome-containing, molybdenum-containing, or tungsten-containing material, for example) deposited on a glass or quartz plate. The reticle is patterned to contain a xe2x80x9chard copyxe2x80x9d of the individual device structure pattern to be recreated on the masking layer overlying a semiconductor structure.
A reticle may be created by a number of different techniques, depending on the method of writing the pattern on the reticle. Due to the dimensional requirements of today""s semiconductor structures, the writing method is generally with a laser or e-beam. A typical process for forming a reticle may include: providing a glass or quartz plate, depositing a chrome-containing layer on the glass or quartz surface, depositing an antireflective coating (ARC) over the chrome-containing layer, applying a photoresist layer over the ARC layer, direct writing on the photoresist layer to form a desired pattern, developing the pattern in the photoresist layer, etching the pattern into the chrome layer, and removing the residual photoresist layer. When the area of the photoresist layer contacted by the writing radiation becomes easier to remove during development, the photoresist is referred to as a positive-working photoresist. When the area of the photoresist layer contacted by the writing radiation becomes more difficult to remove during development, the photoresist is referred to as a negative-working photoresist. Advanced reticle manufacturing materials frequently include combinations of layers of materials selected from chromium, chromium oxide, chromium oxynitride, molybdenum, molybdenum silicide, and molybdenum tungsten silicide, for example.
As previously mentioned, the reticle or photomask is used to transfer a pattern to an underlying photoresist, where the reticle is exposed to blanket radiation which passes through open areas of the reticle onto the surface of the photoresist. The photoresist is then developed and used to transfer the pattern to an underlying semiconductor structure. Due to present day pattern dimensional requirements, which are commonly less than 0.3 xcexcm, the photoresist is preferably a chemically amplified DUV photoresist. In the making of the reticle itself, a chemically amplified DUV photoresist has been used in combination with a direct write electron beam writing tool. However, the exposed, imaged photoresist on the surface of the unpatterned reticle frequently exhibits a xe2x80x9cfootxe2x80x9d at the bottom of the pattern profile, where the photoresist layer interfaces with an underlying ARC layer on a chrome-containing surface, for example, despite the presence of the underlying ARC layer (which is typically a chrome oxynitride material). The foot is not uniform in size across the reticle substrate surface because the basisity changes somewhat randomly across the substrate surface. Since the foot is variable, it makes it difficult to do the metrology which is used to determine whether the finished reticle will meet dimensional requirements. Some imaged and developed positive tone photoresists exhibit a xe2x80x9ctxe2x80x9d-top profile. In addition, the surface of the patterned photoresist layer typically exhibits standing waves, due to reflections which occur during the direct writing on the photoresist layer, despite the presence of the underlying ARC layer.
In their 1992 paper in Microelectronic Engineering (Vol. 17 (1992) 275-278), Gilles Amblard et al. describe how the development of chemically amplified (CA) resist systems has been the most successfil approach to meeting the challenge of high resolution and high speed, posed by X-Ray, Electron-Beam or Deep UV lithography. However, they discovered that pattern profile abnormalities appear which limit the use of a negative resist. Even though the correct exposure dose is applied throughout the thickness of the desired pattern, an aqueous developer dissolves the bottom part of the resist in contact with or near the underlying substrate. Fissures as thick as 0.1 to 0.2 xcexcm were observed in the pattern at the interface with the substrate, resulting in a loss of adhesion in fine patterns. The problem was observed for resists imaged and developed on both spin on glass (SOG) and aluminum substrates. With regard to the aluminum, they observed that because of the amphoteric behavior of aluminum, the acid molecules of the photoresist react when they come in contact with the substrate, thus generating a concentration gradient within the resist material. Because of the lack of acid molecules near the resist/aluminum interface, crosslinking of the resist could not be achieved, and the unreacted resist was washed away during development. A recommended method of overcoming this problem, for a Shipley SAL 603 photoresist imaged using an Electron-beam 20 KeV, is to deposit a layer of Al2O3 or titanium over the aluminum substrate prior to applying the photoresist.
Japanese Patent No. 10048831 assigned to Sony Corp and granted Feb. 20, 1998, relates to patterning of a chemical amplification-based resist film on a film which is to be patterned. The composition of the film to be patterned is not specified in the English abstract of the Japanese patent. The formation process comprises: (a) covering the film to be patterned with a protective coating consisting of chalcogen except sulphur; (b) depositing the chemical amplification-based resist film on the protective coating; (c) applying selective exposure, baking after exposure, and development to the chemical amplification-based resist film to form a resist pattern; and (d) selectively removing the exposed portion of the protective coating. The advantage is said to be that the surface of the film to be patterned is previously passivated by the protective coating. This prevents diffusion of active species between the chemical amplification-based resist film and the film to be patterned and prevents the active species from a decrease in its concentration around the interface against the film to be patterned. The resulting resist pattern is said to have xe2x80x9cno unusual shapexe2x80x9d.
International Application WO99/53378 of S. Funato et al., assigned to Clariant Int. Ltd., published Oct. 21, 1999, describes a method of forming a pattern in a photosensitive film made from a chemical amplification resist material. The method is said to provide high resolution and high precision by preventing reaction products from being formed at the interface between an anitreflection film and a photosensitive material film. This is accomplished by decreasing residuals of the etched film after etching. A film is formed on a semiconductor substrate (which is polysilicon). The film is a photosensitive material made of a chemical amplification resist material containing an onium salt compound and at least either a sulfone compound or a sulfonate. The photosensitive material film is exposed to light through a mask and is developed to form a patterned resist film. Subsequently, the antireflection film is dry etched using an SO2xe2x80x94O2 gas mixture and the polysilicon is dry etched using the patterned resist film.
European Patent Application EP 0 905 565 A1 of Lu Zhijian, assigned to Siemens Aktiengesellschaft, published Mar. 31, 1999, discloses a method for making an anti-reflective layer for improving photoresist resolution and process window. The method includes providing a first volume of an organic anti-reflective chemical and providing a second volume of photo-acid generator chemical. The second volume is between about 0.01 percent and about 30 percent of the first volume. The first volume of organic anti-reflective chemical and the second volume of photo-acid generator chemical are then mixed substantially simultaneously to produce an enhanced anti-reflective chemical which will provide an increased level of acid under exposed photoresist regions. The increase in level of acid in the enhanced anti-reflective chemical is said to minimize the effects of acid loss from a subsequently spin-coated deep ultra-violet photoresist layer at the interface with the anti-reflective coating. The enhanced anti-reflective layer is applied over a semiconductor substrate where the exposed surface may be a dielectric layer, aluminum, copper, or polysilicon. An example given for the photo-acid generator is a diphenyliodonium salt. After application of a DUV photoresist over the enhanced anti-reflective layer, the wafer is subjected to a post-apply bake to drive off solvent and harden the photoresist. The photoresist is exposed to DUV wavelength light (e.g., 248 nm or 193 nm) through a reticle which includes the pattern to be transferred.
U.S. Pat. No. 5,879,863 to Azuma et al., issued Mar. 9, 1999, discloses a method of forming a pattern on a semiconductor substrate which includes forming a film on a semiconductor substrate; bringing a vapor of a first neutralizer into contact with the surface of the film, to form a primer layer of the first neutralizer on the film, where the first neutralizer generates an acid on exposure to light which neutralizes a base species; applying a chemical amplification resist over the primer layer; and selectively exposing the resist layer to light, followed by developing the resist pattern. This method is said to be effective when the film is formed of a material containing a base species. Examples are borophospho silicate glass film or titanium nitride film. An example of the first neutralizer is 2-sulfonyl butane or methyl methacrylate. The chemical amplification resist is said to be a positive resist. In the alternative, when the film to be patterned is acidic, the first neutralizer generates a nucleophile upon exposure to light to neutralize an acid species, or the first neutralizer is weakly basic and neutralizes an acid species by itself. An example of the first neutralizer is acrylamide or pyridine.
U.S. Pat. No. 5,939,236 of Pavelcheck et al, assigned to Shipley Company, L.L.C., issued Aug. 17, 1999, describes a light absorbing crosslinking composition suitable for use as an antireflective composition, particularly for deep UV applications. The antireflective compositions include a photoacid generator that is activated during exposure of an overcoated photoresist. The antireflective composition is said to include a resin binder, an acid or thermal acid generator, and a photoacid generator compound. Antireflective compositions of the invention are said to significantly reduce undesired footing of an overcoated resist relief image. The photoresist is used to transfer an image to a substrate which is exposed through a photomask.
In their paper entitled: xe2x80x9cImprovement of Post Exposure Delay Stability Of Chemically Amplified Positive Resistxe2x80x9d, presented at the SPIE Symposium on Photomask and X-Ray Technology VI, Yokohama Japan, September 1999 (SPIE) Vol. 3748. 0277786X/99, Kohji Katoh et al. describe the development of a novolak-based chemically amplified positive resist for next generation photomask (below 0.18 xcexcm) fabrication. The resist is said to prevent footing profile by the use of a hydrophilic polyphenol compound. The resist was used to make a well defined 0.25 xcexcm line-and-space pattern on a CrOx substrate at a dose of 4.0 xcexcC/cm2. The advanced high acceleration voltage (50 kV) E-beam writer HL-800M was developed to provide better critical dimension control. However, the high acceleration voltage lowers the sensitivity of resists. To compensate, a chemically amplified resist was needed. The resist developed includes four components: a novolak matrix resin, a polyphenol compound, an acid generator, and a dissolution inhibitor.
In their paper xe2x80x9cEnhancement or Reduction of Catalytic Dissolution Reaction in Chemically Amplified Resists by Substrate Contaminantsxe2x80x9d (published in IEEE Transactions On Semiconductor Manufacturing, Vol. 12, No. 4, November 1999), Choi Pheng Soo et al. describe the chemical interaction of resist and substrate at the interface, which modifies the dissolution reaction, and has degraded sidewall profile of the resist features. Depending on the nature of the residue on the substrate, the xe2x80x9cbottom pinchingxe2x80x9d (BP) effect and footing are observed, especially for negative chemically amplified (CA) resists. The BP effect is observed for CA resist on top of an organic bottom antireflection coating (BARC). The BP is attributed to the acid generated from the underlying organic BARC. With optimization on soft bake temperature of BARC, the BP effect is said to be eliminated. On a silicon nitride surface, new chemical information is said to have explained xe2x80x9cfootingxe2x80x9d and BP effects in CA resists. Residual alkaline molecules on the nitride surface are said to play a major role in the formation of footing for a positive tone resist and pinching for a negative tone resist. Less severe footing is said to be observed if the nitride surface is plasma-deposited with a thin oxide cap, which suppresses the surface basicity. However, extended plasma deposition is said to cause surface acidity of a newly formed oxide cap, so that the nitride surface becomes acidic, causing BP.
European Patent Application No. EP0 987 600 A1 of Timothy G. Adams et al., assigned to Shipley Company LLC, published Mar. 22, 2000, describes new light absorbing crosslinking compositions suitable for use as an antireflective composition (ARC), particularly suitable for short wavelength imaging applications such as 193 nm. The ARCs are preferably used with an overcoated resist layer (i.e., as bottom layer ARCs) and in general comprise ARC resin binders that can effectively absorb reflected sub-200 nm exposure radiation. In particular, the antireflective composition comprises a resin binder that has phenol groups. The phenol groups are described as directly pendant from the resin backbone of the antireflective composition resin.
International Patent Application PCT/US00/06314 of Patrick Foster et al., assigned to Arch Specialty Chemicals, Inc., published Sep. 14, 2000, describes the use of chemically amplified bilayer resist systems for deep UV lithography in semiconductor manufacturing. The problem with using deep UV wavelengths is said to be that resists used at the higher wavelengths were too absorbent and insensitive. Thus, in order to utilize deep UV light wavelengths, new resist materials with low optical absorption and enhanced sensitivities were needed. However, chemically amplified resist systems have many shortcomings. One problem is said to be standing wave effects, which occur when monochromatic deep UV light is reflected off the surface of a reflective substrate during exposure. The formation of standing waves in the resist reduces resolution and causes line width variations. Standing waves in a positive resist are said to have a tendency to result in a foot at the resist/substrate interface, reducing the resolution of the resist. In addition, chemically amplified resist profiles and resolution may change due to substrate poisoning. Particularly, this effect is said to occur when the substrate has a nitride layer. It is believed that the Nxe2x80x94H bond in the nitride film deactivates the acid at the nitride/resist interface. For a positive resist, this results in an insoluble area, and either resist scumming, or a foot at the resist/substrate interface. Foster et al. recommend utilizing an underlayer or undercoat film that is placed on the substrate before the chemically amplified film is applied as a means of reducing the above-mentioned problems. A typical bilayer resist process provides for application of the undercoat layer on the substrate, with the chemically amplified resist being applied over the undercoat layer. The invention is said to be a thermally curable polymer composition comprising a hydroxyl-containing polymer, an amino cross-linking agent and a thermal acid generator. The hydroxyl-containing polymer is said to comprise monomer units selected from the group consisting of cyclohexanol, hydroxystyrene, hydroxyalkyl acrylate or methacrylate, hydroxycycloalkyl acrylate or methacrylate, hydroxyalkylcycloalkyl acrylate or methacrylate, arylalkyl alcohol, and allyl alcohol.
U.S. Pat. No. 6,156,479 to Meador et al., assigned to Brewer Science Inc., and issued Dec. 5, 2000 pertains to thermosetting anti-reflective coating compositions for use in multilayer photoresist systems. The anti-reflective coating compositions are said to have improved etch rate, and to be prepared from certain acrylic polymers and copolymers, such as, glycidyl methacrylate reacted with non-polycyclic carboxylic acid dyes and non-polycyclic phenolic dyes, all light absorbing at a wavelength of 193 nm. In describing the prior art, the inventors mention that previously described antireflective coatings may intermix with photoresists upon application of the photoresist, and, that this intermixing produces small but discernible distortions at the bottom of resist features. For feature sizes below 0.3 xcexcm dimensions, even these small distortions become unacceptable for producing good quality, practical devices. Thermosetting antireflective coatings are said to be preferable, but may be difficult to etch, hampering their removal after development of an overlying DUV photoresist.
The above descriptions pertain to the use of chemically amplified photoresist on semiconductor substrates, or to the use of a chemically amplified photoresist in combination with electron beam lithography to produce a reticle. The present invention is different in that it pertains to the use of an optical system, a direct write continuous wave laser, to image a chemically amplified photoresist which is used to transfer a pattern to a photomask (reticle). However, many of the problems described above are experienced in producing a reticle using an optical imaging system in combination with a chemically amplified photoresist.
FIG. 1A shows a schematic of a cross-sectional view of a prior art starting structure 100 used to form a reticle, including, from bottom to top, a quartz substrate 102, overlaid with chrome-containing layer 104, overlaid with an ARC layer 106, and a positive tone photoresist layer 108. As shown in FIGS. 1B and 1C, after patterning of the photoresist layer 108 using an electron-beam writing tool, there is often a xe2x80x9cfootxe2x80x9d 110 extending from the lower portion of patterned photoresist layer 108 toward the surface 116 of ARC layer 106. The presence of a foot (feet) 110 makes it difficult to maintain control of the critical dimensions during subsequent etch transferring of the photoresist pattern through the ARC layer 106 and chrome containing layer 104. The foot also impacts the metrology capabilities of the lithographer.
FIG. 1C, which is an enlargement (from FIG. 1B) of a portion of the patterned photoresist layer 108 (with underlying ARC layer 106), shows a line 107 which exhibits xe2x80x9ctxe2x80x9d-topping 113 in the upper portion of line 107, feet 110 at the base of line 107, and ripples (standing waves) 114 on the sidewall 111 surfaces 112 of line 107. The xe2x80x9ctxe2x80x9d-topping 113 is believed to be caused by contamination/reaction which occurs at the upper surface of the photoresist layer during processing prior to development of the pattern. The standing waves 114 are generated by reflected radiation within the photoresist material, which occurs during the direct writing of the pattern into photoresist layer 108 by the electron-beam writing tool. The ARC layer 106 helps reduce the standing wave effect by reducing reflection back from underlying layers and device features into the photoresist layer 108, but standing waves are generated in varying degrees depending on the imaging system and the material composition of the particular photoresist. When the photoresist is a chemically amplified photoresist, transparency of the photoresist material is particularly high throughout the entire direct writing process; this results in increased reflectivity (greater than that for earlier i-line novolak photoresists), which increases the formation of standing waves 114.
Thus, it would be highly desirable to have a method of making a photomask which provides features having critical dimensions of 0.3 xcexcm or less. To accomplish this, it is necessary to have a method of producing a patterned, developed photoresist which exhibits minimal surface distortions in the form of feet at the base, xe2x80x9ctxe2x80x9d-topping at the top of the resist, and standing waves along the sidewalls of the developed photoresist. This improved developed photoresist can be used to transfer the pattern for the feature to an underlying photomask (reticle).
One embodiment of the present invention pertains to a method of optically fabricating a photomask using a direct write continuous wave laser, which includes the steps of applying an organic antireflection coating over a metal-containing layer; applying a chemically-amplified DUV photoresist, either positive tone or negative tone, over the organic antireflection coating; and exposing a surface of the DUV photoresist to radiation from the direct write continuous wave laser. Preferably the laser is operated at 244 nm or 257 nm. The metal-containing layer may include combinations of layers of materials selected from chromium, chromium oxide, chromium oxynitride, molybdenum, molybdenum silicide, and molybdenum tungsten silicide, for example and not by way of limitation. The organic antireflection coating may be selected from a negative photoresist containing a DUV dye; a polymeric material prepared from acrylic polymers or copolymers; a binder resin combined with an acid or thermal acid generator and a photoacid generator compound; a binder resin having pendent phenyl groups; and combinations thereof.
The organic anti-reflective coating composition preferably comprises acrylic polymers and/or copolymers.
In an alternative embodiment of the method of fabricating a photomask, the organic antireflection-coating is applied over an inorganic antireflection coating. The inorganic antireflection coating may be selected to include a material such as chrome oxynitride, titanium nitride, silicon nitride or molybdenum silicide.